Thunderstorm Weather 

The hydrometers and turbulence of a thunderstorm that we observe and record at the surface are easily recognized. The weather within the thundercloud itself is another story. Visual observations from aircraft are difficult because of the speed with which they pass through the thunderclouds, and man has yet to devise an in-strument that will measure all hydrometers in the cloud. Let us look at those forms of precipita-tion turbulence and icing occurring with and within thunderclouds as we know them today.

RAIN. Liquid water in a storm may be ascending if encountered in a strong updraft; it may be suspended, seemingly without motion, yet in an extremely heavy concentration; or it may be falling to the ground. Rain, as normally measured by surface instruments, is associated with the downdraft. This does not preclude the possibility of a pilot entering a cloud and being swamped, so to speak, even though rain has not been observed from surface positions. Rain is found in almost every case below the freezing level. In instances in which no rain is encountered, the storm probably has not developed into the mature stage. Statistics show that although heavy rain is generally reported at all levels of a mature storm, the greatest incidence of heavy rain occurs in the middle and lower levels of a storm.

HAIL. Hail, if present, is most often found in the mature stage. Very seldom is it found at more than one or two levels within the same storm. When it is observed, its duration is short. The maximum occurrence is at middle levels for all intensities of hail.

SNOW. The maximum frequency of moderate and heavy snow occurs several thousand feet above the freezing level. Snow, mixed, in many cases, with supercooled rain, may be en-countered in updraft areas at all altitudes above the freezing level. This presents a unique icing problem: wet snow packed on the leading edge of the wing of the aircraft resulting in the formation of rime ice. 

TURBULENCE. There is a definite cor-relation between turbulence and precipitation. The intensity of associated turbulence, in most cases, varies directly with the intensity of the precipitation.

ICING. Icing may be encountered at any level where the temperature is below freezing. Both rime and clear ice occur, with rime predominating in the regions of snow and mixed rain and snow. Since the freezing level is also the zone of greatest frequency of heavy turbulence and generally heavy rainfall, this particular altitude appears to be the most hazardous for aircraft. 

SURFACE WIND. A significant hazard associated with thunderstorm activity is the rapid change in surface wind direction and speed im-mediately before storm passage. The strong winds at the surface accompanying thunderstorm passage are the result of the horizontal spreading out of downdraft current from within the storm as they approach the surface of Earth. The total wind speed is a result of the downdraft divergence plus the forward velocity of the storm cell. Thus, the speeds at the leading edge, as the storm approaches, are greater than those at the trailing edge. The initial wind surge, as observed at the surface, is known as the FIRST GUST.

The speed of the first gust is normally the highest recorded during storm passage, and the direction may vary as much as 180 from the previously prevailing surface wind. First-gust speeds increase to an average of about 16 knots over prevailing speeds, although gusts of over 78 knots (90 mph) have been recorded. The average change of wind direction associated with the first gust is about 40.

In addition to the first gust, other strong, violent, and extremely dangerous downdraft winds are associated with the thunderstorm. These winds are referred to as DOWNBURSTS. Down-bursts are subdivided into MACROBURSTS and MICROBURSTS.

Macrobursts. Macrobursts are larger scale downbursts. Macrobursts can cause widespread damage similar to tornadoes. These damaging winds can last 5 to 20 minutes and reach speeds of 130 knots (150 mph) or more.

Microbursts. Microbursts are smaller scale downbursts. A microburst can last 2 to 5 minutes and can also reach wind speeds in excess of 130 knots. Microbursts produce dangerous tailwinds or crosswinds and windshear for aircraft and are difficult to observe or forecast.

Downbursts are not the same as first gusts. First gusts occur in all convective cells containing showers and are predictable and expected. Downbursts, however, do not occur in all con-vective cells and thunderstorms.

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